1) Field of the Invention
The present invention relates to an optical information recording apparatus for recording optical information in a phase change type optical recording medium and a method therefor, and more particularly to an optical information recording apparatus for recording optical information in a phase change type optical recording medium using two spots of light generated from light sources such as semiconductor lasers and a method therefor.
2) Description of the Related Art
Conventionally, there have been various kinds of methods for recording an information signal in a phase change type optical recording medium by utilizing a reversible phase change, such as a phase change caused between a crystal state (a crystal phase) and an amorphous state (an amorphous phase), and a phase change caused between a crystal state and another crystal state. In the phase change type optical information recording medium, the crystal state corresponds to a relatively high ordered state of the atomic arrangement thereof, and the amorphous state corresponds to a relatively low ordered state of the atomic arrangement thereof.
The fundamental mechanism of each of these methods is that a change in the construction or the state of the atomic arrangement of the material of the optical recording medium is caused by projecting a light thereon. However, these methods are not classified as a so-called photon mode recording method for recording optical information in an optical recording medium such as a silver salt photographic film, a photochromic recording thin film, by causing a chemical reaction. They are classified as a so-called heat mode recording method for recording optical information in an optical recording medium by converting an absorbed light into heat, resulting in an increase in the temperature of the optical recording medium, and further causing a thermal transformation.
For example, in the case of recording optical information in a recording thin film of an optical recording medium of a chalcogenide glass using a semiconductor laser, the following optical recording method is used.
Namely, as soon as the temperature of a recording portion by projecting a beam of laser light on the recording portion of the recording thin film is increased, the beam of laser light is moved from the recording portion to the other portion thereof, or the light power of the laser light is decreased. At that time, the heat locally caused in the recording portion due to the light absorption diffuses toward the peripheral portions of the recording portion at a high speed, and then the temperature of the recording portion decreases or the recording portion is cooled. Then, the maximum temperature of the recording portion and the cooling speed thereof can be controlled by properly selecting the light power of the laser light and the time of the laser light projection, etc. An information signal can be recorded in the optical recording medium, be reproduced therefrom, and be erased therefrom by utilizing changes of optical characteristics of the optical recording medium caused after cooling the recording portion, such as the reflectance and the transmittance, depending on the above-mentioned condition of the laser light projection.
More concretely, after a recording portion of the recording thin film is melted instantaneously by projecting a beam of laser light thereon, an amorphous state of the recording thin film can be obtained under a rapid cooling condition, and also a crystal state thereof can be obtained under a gradual cooling condition. Further, a crystal state thereof can be obtained by annealing a recording portion thereof without melting an amorphous state of the solid phase thereof.
A method for recording and erasing an information signal by projecting a beam of laser light on the above-mentioned recording medium which rotates or moves on a straight line is classified into the following two methods. One is a method for overwriting an information signal by modulating the light power of a beam of laser light between a recording level, such as an amorphizing level, and an erasing level, such as a crystallizing level. Another is a method for overwriting an information signal by projecting plural beams of laser lights in a manner similar to that of the former method. It is to be noted that the former method is disclosed in the Japanese patent laid-open publication (JP-A) No. 56-145530, and the latter method is disclosed in the Japanese patent laid-open publication (JP-A) No. 59-71140.
The former method has the advantage that the composition of the optical system is extremely simple since there is used only one laser system for projecting a beam of laser light on a recording thin film of an optical recording medium. However, since the time of projecting a beam of laser light on the recording thin film is determined depending on the relative moving speed between a circular spot of laser light whose diameter is as large as about one micron and the optical recording medium, there is the problem that the range of selecting the material of the thin recording film is limited by the recording speed of the optical recording system to be applied. For example, in the case of recording a large amount of data, such as image information in an optical recording medium, it is necessary to set a large relative moving speed between the spot of laser light and the optical recording medium, at about several tens m/sec., resulting in an inevitable decrease in the time of the laser light projection.
In order to solve the above-mentioned problems, it is necessary to provide an optical recording medium of a recording material the crystallization of which can be completed within a relatively short heating time interval, namely, to provide a material having a large crystallizing speed. The shortest crystallizing time of the above-mentioned phase change type optical recording material among those which have been reported up to now is about 50 nsec. It is supposed that when the relative moving speed is heightened, the time of the laser light projection may reach a lower limit of the time required for crystallizing the optical recording medium. Namely, there is the problem that an upper limit of the recording speed is determined depending on the projection method itself.
The latter method includes a fundamental method for projecting a circular spot of laser light having a relatively large power density upon recording an information signal in an optical recording medium or amorphizing the optical recording medium, and for projecting a thin and long spot of laser light having a relatively small power density upon erasing the recorded information signal therefrom or crystallizing the optical recording medium. In this case, for example, there can be used such a condition that the longitudinal length of the spot of laser light for erasing a recorded information signal is set at ten microns and the time of the laser light projection is set at about ten times the time upon recording an information signal in the optical recording medium. Therefore, the latter method has such an advantage that the kind and composition of the recording film of the optical recording medium can be relatively freely selected. However, it is to be noted that the optical system becomes complicated, since it is necessary to form a beam of laser light in the shape of a thin and long spot, and the servo control system becomes complicated since it is necessary to track the thin and long spot of laser light on a track line of the optical recording medium.
In the case of forming a recording mark in the optical recording medium by crystallizing it in order to increase the speed of recording, for example, there is used a preparatory heating beam of laser light prior to a recording beam of laser light so as to previously increase the temperature of the recording beam of laser light so as to previously increase the temperature of the recording portion up to a temperature higher than the room temperature. However, in the case of overwriting an information signal on the optical recording medium, there is the problem that it is difficult to record an information signal therein. Namely, when a portion of the optical recording medium to be recorded is previously heated, a thermal gradient between the center of the recording portion and the peripheral portions thereof becomes small as soon as an information signal is recorded therein. Therefore, the heat caused in the recording portion does not easily diffuse toward the peripheral portions thereof. As a result, there is a problem in that the recording portion of the optical recording medium can not be amorphized under a predetermined rapid cooling condition. Further, there is such a problem that the boundary between an amorphous mark portion and a non-mark portion of the optical recording medium is not distinctly formed, resulting in a decrease in the level of the information signal to be read out from the optical recording medium.